The present invention relates to an electrostatic actuator having a cantilever electrode and a method for producing the same.
Normally, an electrostatic actuator is structured as follows, here described exemplarily from a bottom to a top in the following order: a fixed or movable first electrode is disposed on a first actuator material and covered by an isolation layer. On the latter, a sacrificial layer is disposed, which is again covered by a fixed or movable second electrode. A second actuator material is disposed on the second electrode. The material of the electrodes can differ from the actuator material, both in the first and the second electrode, wherein also the first actuator material can differ from the second actuator material, in particular, the first or second actuator material can be a substrate on which the actuator is formed in an unmovable manner. After the layers and electrodes are joined, the sacrificial layer is removed, whereupon a gap remains between the electrodes within the structure.
Common production processes of electrostatic actuators result in an air gap between the two electrodes of an electrostatic actuator, among others due to the introduced sacrificial layers. The distance between the electrodes resulting from the gap leads, on the one hand, to a significant increase of the electric voltage necessitated for operating the actuator, on the other hand to a change in the deflection behavior of the actuator in dependence on the currently obtained deflection state, up to the “pull-in” effect.
The generated force of an electrostatic actuator decreases quadratically with an increasing distance of the electrodes to one another. The force of the electrostatic actuator for the case of the classical parallel-plate actuator can be given, by neglecting the isolation layer, by the equationF=½(e0AV2)/(g−d)2,wherein F describes the electrostatic force, e0 the electric field constant, V the applied voltage, d the voltage-dependent deflection, A the capacitor area and g the initial distance of the electrodes to one another. Due to the geometry of the electrostatic actuator and the highly nonlinear force generation, depending quadratically on the distance of the electrodes to one another with the ratio
      1                  (                  g          -          d                )            2        ,the efficiency of an electrostatic actuator significantly decreases with increasing distance of the electrodes to one another. In the case of a parallel-plate actuator, this distance is the maximum possible travel.
FIG. 3a shows a side view of an electrostatic actuator according to conventional technology, wherein a stationary electrode 14 is disposed on a substrate 12, and an isolation layer 16 is disposed on a side of the stationary electrode 14 facing away from the substrate 12. Opposite to the stationary electrode 14, a bender 32 fixed in a fixed clamping 18 is disposed. A movable electrode 34 is disposed on a side of the bender 32 facing the stationary electrode 14, wherein a gap 17 is formed between the movable electrode 34 and the isolation layer 16, wherein the gap 17 is, for example, a result of removing a sacrificial layer which has been disposed during the above-described production process of the actuator.
That positioning of the two electrodes 13 and 34 to one another with the gap 17 formed between the electrodes 14 and 34 results in the above-described limitations of the actuator.
FIG. 3b shows a top view of the electrostatic actuator of FIG. 3a. The bender 32 disposed on the fixed clamping 18 is implemented in a planar uniform manner.
Both the gap 17 between the electrodes 14 and 34 and the nonlinear force generation result in a pull-in effect when exceeding a control voltage between electrodes 14 and 34 depending on geometry and material parameters.
Due to production-induced limitations, the gap 17 between the electrodes 14 and 34 cannot be completely closed, but is minimized by means of optimizing the production process. Thereby, the loss of electrostatically generatable and usable force caused by the remaining distance of the electrodes 14 and 34 to one another is accepted.
Conventional technology includes a great number of different configurations of electrostatic actuators. Known realizations show, for example, benders or plates that are cantilevered via a spring element or clamped on both sides by fixed clampings or spring elements. Also concerning the geometry of stationary electrode or bender, a great number of different realizations is known, wherein the gap between the electrodes leads to the above described disadvantages in the realizations.
Publications describe how the gap between the electrodes can be partially closed by means of a very soft tab contained by the bender by electrostatic excitation very early, i.e. already at a very low applied electric voltage, by effecting deflection of the soft tab towards the opposing electrode by attractive force between the electrodes caused by the applied voltage, and the deflection partially closes a gap at a contact point. The gap itself, however, still remains.